LWG4445 sch_ must not be in moved-from state

Fixes NB US 239-367 (C++26 CD).

Author: burblebee

source/exec.tex

@@ -7001,6 +7001,9 @@
         && @\libconcept{scheduler}@<Sch>
     explicit task_scheduler(Sch&& sch, Allocator alloc = {});
 
+    task_scheduler(const task_scheduler&) = default;
+    task_scheduler& operator=(const task_scheduler&) = default;
+
     @\exposid{ts-sender}@ schedule();
 
     friend bool operator==(const task_scheduler& lhs, const task_scheduler& rhs)